Optical sensor for measuring physiological properties

ABSTRACT

The invention provides a physiological probe that comfortably attaches to the base of the patient&#39;s thumb, thereby freeing up their fingers for conventional activities in a hospital, such as reading and eating. The probe, which comprises a separate cradle module and sensor module, secures to the thumb and measures time-dependent signals corresponding to LEDs operating near 660 and 905 nm. The cradle module, which contains elements subject to wear, is preferably provided as a disposable unit.

RELATED APPLICATIONS

The present application claims the benefit of priority to U.S.Provisional Application No. 61/444,320, filed Feb. 18, 2011, which ishereby incorporated by reference, including the drawings.

BACKGROUND OF THE INVENTION

The following discussion of the background of the invention is merelyprovided to aid the reader in understanding the invention and is notadmitted to describe or constitute prior art to the present invention.

The saturation of peripheral oxygen in the blood (SpO2) is sometimesreferred to as the ‘fifth vital sign’. Medical professionals can detecthypoxemia, i.e. a deficiency of oxygen, by monitoring a patient's SpO2.Values between about 95-100% are considered normal; those below thisindicate hypoxemia, and will typically trigger an alarm in a hospitalsetting.

A technique called pulse oximetry measures SpO2. Technically thisparameter is determined from a patient's arterial oxygen saturation, orSaO2, which is a percentage of oxygenated arterial hemoglobin present intheir blood. Functional hemoglobin molecules can bind with up to fouroxygen molecules to yield ‘oxygenated’ hemoglobin (HbO2). A hemoglobinmolecule bound to less than four oxygen molecules is classified as‘reduced’ hemoglobin (Hb). Conventional pulse oximeters featurealgorithms that assume only HbO2 and Hb are present in the blood, andmeasure SpO2 from the ratio of oxygenated hemoglobin to the total amountof hemoglobin (both oxygenated and reduced) according to equation (1):

$\begin{matrix}{{{Sp}\; O\; 2} = \frac{{Hb}\; O\; 2}{{{Hb}\; O\; 2} + {Hb}}} & (1)\end{matrix}$

HbO2 and Hb feature different absorption spectra in the visible andinfrared regions, and can therefore be measured optically. Conventionalpulse oximeters thus typically feature light sources (most typicallylight-emitting diodes, or LEDs) that radiate in the red (near 660 nm)and infrared (typically between 900-950 nm) spectral regions. Aphotodetector measures a portion of radiation at each wavelength thattransmits through the patient's pulsating blood, but is not absorbed. At660 nm, for example, Hb absorbs about ten times as much radiation asHbO2, whereas at 905 nm HbO2 absorbs about two times as much radiationas Hb. Detection of transmitted radiation at these wavelengths yieldstwo time-dependent waveforms, each called a plethysmogram (PPG), that anoximeter analyzes to solve for SpO2 as defined in equation (1) above.

Specifically, the oximeter processes PPG waveforms measured with red(RED(PPG)) and infrared (IR(PPG)) wavelengths to determinetime-dependent AC and DC signals. The term ‘AC’ signals, as used herein,refers to a portion of a PPG waveform that varies relatively rapidlywith time, e.g. the portion of the signal modulated by pulsations in thepatient's blood. ‘DC’ signals, in contrast, are portions of the PPG thatare relatively invariant with time, e.g. the portion of the signaloriginating from scattering off of components such as bone, skin, andnon-pulsating components of the patient's blood.

More specifically, AC signals are modulated by a heartbeat-induced pulsepresent in both waveforms. The pulse represents a pressure wave,launched by the heart, which propagates through the patient'svasculature and causes a time-dependent increase in volume in botharteries and capillaries. When the pressure pulse reaches vasculatureirradiated by the oximeter's optical system, a temporary volumetricincrease results in a relatively large optical absorption according tothe Beer-Lambert Law. Typically only about 0.5-1% of the total signalmeasured by the photodetector originates from the AC signal, with theremainder originating from the DC signal. Separation of AC and DCsignals is typically done with both analog and digital filteringtechniques that are well-known in the art.

During pulse oximetry a normalized ‘r’ value is typically calculatedfrom AC and DC signals using equation (2), below:

$\begin{matrix}{r = \frac{660\mspace{14mu}{{{nm}({AC})}/660}\mspace{14mu}{{nm}({DC})}}{905\mspace{14mu}{{{nm}({AC})}/905}\mspace{14mu}{{nm}({DC})}}} & (2)\end{matrix}$r, which is sometimes called a ‘ratio of ratios’ (RoR), represents aratio of Hb to HbO2. It equates an actual SpO2 value, which ranges from0-100% O2, to an empirical relationship that resembles a non-linearequation. Above about 70% O2 this equation typically yields values thatare accurate to a few percent. Measurements below this value, while notnecessarily accurate, still indicate a hypoxic patient in need ofmedical attention.

Like SpO2, continuous noninvasive blood pressure (“cNIBP”) monitoringrelies on accurate measurement of PPG and ACC waveforms obtained from apulse oximeter, together with an electrocardiogram waveform (ECG). cNIBPis typically measured with the ‘Composite Technique’, which is describedin detail in the co-pending patent applications entitled: VITAL SIGNMONITOR FOR MEASURING BLOOD PRESSURE USING OPTICAL, ELECTRICAL, ANDPRESSURE WAVEFORMS (U.S. Ser. No. 12/138,194; filed Jun. 12, 2008 andpublished as 20090018453A1), and BODY-WORN SYSTEM FOR MEASURINGCONTINUOUS NON-INVASIVE BLOOD PRESSURE (cNIBP) (U.S. Ser. No.12/650,354, filed Nov. 15, 2009 and published as 20100168589A1), thecontents of which are fully incorporated herein by reference.

As described therein, the Composite Technique (or, alternatively, the‘Hybrid Technique’ referred to therein) typically uses a single PPGwaveform from the SpO2 measurement (typically the IR(PPG) waveform, asthis typically has a better signal-to-noise ratio than the RED(PPG)waveform), along with the ECG waveform, to calculate a parameter called‘pulse transit time’ (PTT) which strongly correlates to blood pressure.Specifically, the ECG waveform features a sharply peaked QRS complexthat indicates depolarization of the heart's left ventricle, and,informally, provides a time-dependent marker of a heart beat. PTT is thetime separating the peak of the QRS complex and the onset, or ‘foot’, ofthe RED/IR(PPG) waveforms; it is typically a few hundred milliseconds.The QRS complex, along with the foot of each pulse in the RED/IR(PPG),can be used to more accurately extract AC signals using a mathematicaltechnique described in detail below. In certain embodiments, both theRED/IR(PPG) waveforms may be collectively processed to enhance theaccuracy of the cNIBP measurement.

Typical pulse oximeters feature a probe encased in a clothespin-shapedhousing that includes both red and infrared LEDs, and a photodetectorthat detects radiation from the LEDs after it passes through a portionof the patient's body. The probe typically clips to a patient's indexfinger. Most probes operate in a transmission-mode optical geometry, andrelay analog waveforms measured by LEDs and the photodetector to anexternal processing unit. Because it is based on an optical measurement,pulse oximetry can be extremely sensitive to a patient's motion.Activities such as walking, finger tapping, falling, and convulsing canresult in a number of artifacts that distort both the AC and DCcomponents of waveforms measured with the oximeter's optical system.Motion-related activities, for example, can cause the oximeter probe tomove relative to the patient's finger, change the amount of ambientlight that irradiates the photodetector, and disrupt both arterial andvenus blood flow in vasculature measured by the optical system. Each ofthese events can generate artifacts that, in some cases, are similar tothe AC and DC signals within the PPG waveforms. Ultimately this cancause the pulse oximeter to generate inaccurate values and false alarms.

International Patent Application No. PCT/US2010/039000, which is herebyincorporated by reference in its entirety, describes a physiologicalprobe that comfortably clips to the base of the patient's thumb, therebyfreeing up their fingers for conventional activities in a hospital, suchas reading and eating. The probe reversibly secures to the thumb with,e.g., an easy-to-use Velcro strap, disposable tape, or similar closure,or may be provided in the form of a closed ring which slips over thethumb. It measures time-dependent waveforms (RED/IR(PPG)) correspondingto LEDs typically operating near 660 nm and 905 nm. Clinically accuratepulse oximetry measurements made at the base of the patient's thumbrequire a set of coefficients relating r (from Eq. 2) to SpO2 that aretypically determined with a set of empirical experiments (e.g. a‘breathe down’ study, described below). These coefficients differ fromthose used in conventional oximetry measurements because of thedifferences between vasculature in the base of the thumb and the tip ofthe index finger. Typically the base of the thumb features relativelyfewer capillary beds, and thus the coefficients are preferably adjustedaccordingly.

It is to be understood that the invention is not limited in itsapplication to the details of construction and to the arrangements ofthe components set forth in the following description or illustrated inthe drawings. The invention is capable of embodiments in addition tothose described and of being practiced and carried out in various ways.Also, it is to be understood that the phraseology and terminologyemployed herein, as well as the abstract, are for the purpose ofdescription and should not be regarded as limiting.

As such, those skilled in the art will appreciate that the conceptionupon which this disclosure is based may readily be utilized as a basisfor the designing of other structures, methods and systems for carryingout the several purposes of the present invention. It is important,therefore, that the claims be regarded as including such equivalentconstructions insofar as they do not depart from the spirit and scope ofthe present invention.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a physiologicalprobe having at least one emitter and an associated detector configuredto attach to a tissue site, where the detector is configured to measurea detector signal responsive to the intensity of energy from the emitterafter it has interacted with a tissue site. These physiological probes,which are well suited to function as pulse oximeter probes as describedhereinafter, are configured for securing to a digit of the subject, andmost preferably to a subject's thumb at the level of a proximal phalanx.

In particular, the physiological probe of the present inventioncomprises a sensor module and a cradle module. In various aspects, thepresent invention relates to the individual components of the probe, tothe probe itself, and to methods of its manufacture and use.

The sensor module comprises: (a) a electronic circuitry, which incertain embodiments takes the form of a flexible circuit board, whichcomprises (i) one or more, and preferably at least two sources ofelectromagnetic radiation, and (ii) a photodetector configured to detectthe radiation from the source(s) which has passed through or beenreflected by the subject's tissue, thereby acquiring data relating to atleast the oxygen saturation level of blood of the subject, wherein thephotodetector is operably connected to a connection cable for deliveryof photodetector signals to an external processing unit; and (b) aflexible enclosure providing a cover for the circuit board, wherein theflexible enclosure comprises (i) at least one aperture through which thesource(s) emit radiation for irradiating the subject's tissue, (ii) atleast one aperture through which the photodetector receives radiationwhich has interacted with (e.g., passed through and/or been reflectedby) the subject's tissue, and (iii) an aperture through which theconnection cable passes.

The cradle module comprises: at least first and second rigid housingmembers mated to one another via a hinge region to form an approximatelysemicircular ring which is configured to reversibly receive the sensormodule; that is, in use the sensor module is releasably attached to thecradle module.

Optionally, the physiological probe of the present invention furthercomprises a retainer attached to the cradle module and configured toreversibly attach the physiological probe to the digit of the subject,for example by wrapping around the entire circumference of the digit.

As used herein, the term “releasably attached” refers to two separatemodules which may be engaged with and disengaged from one another in thecourse of normal use. In certain embodiments, the hinge region isconfigured to provide a range of adjustment in the diameter of the ringwhile physically constraining the included angle measured between thesources and the photodetector within a predetermined range. As usedherein, the term “semicircular ring” is not meant to refer to thegeometric shape forming an arc of constant radius extending through 180degrees. Rather, the term refers to a shape which fits circumferentiallyaround a portion of, but not the complete circumference of, a digit asdepicted, for example, in FIG. 4.

As noted, the hinge region provides a range of adjustment in thediameter of the semicircular ring formed by the cradle, thereby allowingthe physiological probe to adjust to a range of different digit girths.In certain embodiments meant to be compatible with the human thumb, thehinge of an adult-sized cradle is preferably designed to accommodate agirth of between about 6.2 cm and about 7.4 cm, with pediatric sizes orsizes for other (smaller) digits being made appropriately smaller. Atthe same time, it is important for proper functioning that the geometryof the radiation sources and the photodetector be maintained within anacceptable range for proper function of the sensor.

In one example, the hinge region may be a pivoting joint in which a pinon one rigid housing member fits into a corresponding hole on the otherrigid housing member. Alternatively, the hinge region may simply be aflexible region which bridges the two rigid housing members. Other typesof hinge arrangements will be apparent to the skilled artisan. Whendesired, the angular constraint can be accomplished by providing “stops”in one or both of the first and second rigid housing members whichprevent opening or closing of the hinge past the predetermined range.Preferably, the predetermined range of the included angle measuredbetween the sources and the photodetector are less than or equal toabout 60°, and more preferably between about 60° and about 30°, and mostpreferably between about 55° and about 35°. The term “about” in thecontext of this patent application refers to +/−10% of a givenmeasurement.

The cradle module is preferably designed as a disposable component whichreceives a sensor module preferably designed for multiple uses. As usedherein, the term “disposable” with regard to the cradle refers to thecharacteristic that the cradle module may be disengaged from sensormodule in the course of normal use by the user of the physiologicalprobe such that the sensor module may be easily separated from, and neednot be discarded with, the cradle. This can serve to place the devicecomponents of the physiological probe most susceptible to wear andcleanability issues on a disposable unit, while retaining the moreexpensive electronic components on an easily cleanable and reusableunit. In certain embodiments, the hinge is configured such that a forcemay be applied to move the hinge past one or more of its “stops,”thereby separating the first and second rigid housing members andpreventing improper reuse. This can also facilitate removal of thesensor module for its reuse. In certain embodiments, the sensor modulecan be rendered cleanable for reuse by sealing of the aperturespositioned proximal to the radiation source(s) and the detector with amaterial providing sufficient transparency to the appropriatewavelengths being employed in the device.

When the sensor module is mated to the cradle, the sensor modulesubstantially conforms to the semicircular ring shape created by thecradle. As used herein, the term “substantially conforms” refers to amodule which fits into the designed shape of another module in a mannerwhich permits the two modules to function as a unit in the intendedfashion. To facilitate this conformal fit, the sensor module maycomprise one or more flexible hinge regions, (e.g., one or more livinghinges molded into the flexible enclosure).

In certain embodiments, the surface of the sensor module and the surfaceof the cradle module which come into contact when the two modules aremated comprise corresponding registration structures, e.g., a ridgestructure on one module which inserts into a corresponding dimplestructure on the other module; a tab on one module which fits into acorresponding slot on the other module. The term “registration” as usedherein refers to the act of adjusting or aligning the parts of a devicein relation to each other. These corresponding registration structurescan help to ensure proper placement of the sensor module into the cradlemodule. The combination of a rigid cradle housing and carefulregistration of components helps to maintain a consistent orientation ofthe optical components in the probe and improves the consistency of theoptical path through the artery and capillary tissue, while maintainingcomfort through the use of relatively soft and flexible materials on thesensor module.

In preferred embodiments, the sensor module comprises at least twoflexible hinge points, such that mating of the sensor module to thecradle creates three substantially planar sensor module surfaces. Afirst planar surface comprises the radiation sources; a middle planarsurface provides a flat base surface to engage the thumb tissue of thesubject; and a third planar surface comprises the photodetector. Thesesubstantially planar surfaces can serve to improve pressure applicationto the underlying arteries, thereby helping to increase signalamplitudes and maintain a consistent light path.

In certain embodiments, the length of the cradle module is less than theaverage length of the desired proximal phalanx. In the case of anadult-sized thumb, the cradle may be about 1 cm to 1.5 cm in length,with pediatric sizes being made appropriately smaller. The term “about”in this context refers to +/−10% of a given measurement.

The radiation sources preferably emit radiation at about 660 nm andabout 905 nm wavelengths. Preferably, the radiation sources are proximalto one another, and most preferably contained within the same electronicpackage. In proper use, the physiological probe is placed on the palmaraspect of the digit, preferably the thumb. The present invention canprovide a consistent optical path from the radiation sources, throughthe princeps pollicis artery and soft tissue below the proximal phalanx,and subsequently to the photodetector.

Still other embodiments are found in the following detailed descriptionof the invention, and in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a three-dimensional mechanical drawing of an exemplarycradle module for use as a component of a physiological probe, designedto function as a pulse oximeter probe, of the present invention.

FIG. 2 shows a three-dimensional mechanical drawing of an exemplarysensor module for use as a component of a physiological probe, designedto function as a pulse oximeter probe, of the present invention.

FIG. 3 shows a photograph of a distal end of the sensor module of FIG. 2which includes a pair of light sources and a photodetector.

FIGS. 4A-B show three-dimensional mechanical drawings of the cradlemodule of FIG. 1 attached to a flexible material configured to wraparound a patient's thumb and housing the sensor module of FIG. 2.

FIG. 4C shows a two-dimensional mechanical drawing of the cradle moduleof FIG. 1 housing the sensor module of FIG. 2.

FIG. 5 is a series of mechanical drawings depicting how the sensormodule of FIG. 2 inserts into the cradle module of FIG. 1 to form anexemplary physiological probe.

FIG. 6 shows a graph depicting a relationship between SpO2 and a ‘ratioof ratios’ (RoR) for measurements from theoretical model in a medicaltextbook, made from the tip of a patient's index finger, and made fromthe base of a patient's thumb.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1-4 depict components of a semicircular physiologic probe 100comprising a separate sensor module 101 and cradle module 102 which mateto one another. The probe is designed to be comfortably worn forextended periods (e.g. several days) while freeing up the patient'sfingers for activities such as reading and eating that are commonplacein, e.g., a hospital. While described in detail as a pulse oximeterprobe, the physiological probes of the present invention are applicableto any similar device having at least one emitter and an associateddetector configured to attach to a tissue site.

The cradle module 102 is shaped as a semicircular ring that wraps arounda base of patient's digit to measure time-dependent waveforms (PPG) thatcan be used, e.g., in the determination of SpO2 and cNIBP. The cradlemodule 102 is comprised of two substantially rigid members 103, 104connected by a hinge 105. The rigid members 103, 104 may be made out ofa number of materials known in the art such as plastics, metals, and thelike; for example, polypropylene, acrylates (PMMA, SMMA, etc.), styrenes(SAN, PS, ASA, ABS, etc.), polycarbonate, copolymers, ionomer resins(Surlyn), polyamides, polyesters, thermoplastic elastomers, aluminum,brass, nickel, etc. A preferred example is Eastman Tritan™ copolyester.The length of the cradle module is less than the average length of theproximal phalanx so as not to interfere with bending of the digit at thejoint. An optimal length for an adult-sized cradle configured forattachment to a thumb is in the range of 1 cm to 1.5 cm.

A portion of each rigid member lying below the hinge location byproviding points of interfering physical contact between the two rigidmembers which act as stops to constrain the opening and closing of thehinge within a predetermined range. This serves two purposes. First, thehinge 105 provides a range of adjustment in the diameter of the openingin the semicircular ring which fits over the digit, thereby allowing thephysiological probe to adjust to a range of different finger sizes. Incertain embodiments, the hinge of an adult-sized cradle is preferablydesigned to accommodate a thumb diameter of about 1.8-2.3 cm, or a girthof between about 6.2 cm and about 7.4 cm. Second, to increase the amountof radiation that passes through the artery and capillaries in thetissue, and thereby optimize signal quality, the cradle acts to maintainthe LEDs 106, 107 and photodiode 108 at an included angle (θ) ofapproximately 35-55 degrees. As shown in FIG. 4C, the ‘included angle θ’refers to the angle formed between a first line drawn parallel to theplanar surface comprising the radiation source and a second line drawnparallel to the planar surface comprising the photodetector. Opticalcomponents separated at this angle tend to increase the relativecontribution of signal coming from the artery; ultimately this improvesthe accuracy of the cNIBP measurement, as PTT values measured fromarterial components correlate better to blood pressure than thosemeasured from capillary components.

The probe makes a transmission-mode optical measurement along an innerportion of the digit with the pair of embedded LEDs 106, 107 operatingat, respectively, 660 and 905 nm, and a single photodetector 108 thatdetects these wavelengths after they pass through vasculature and othertissue lying beneath the LEDs. Preferably, both LEDs and thephotodetector are positioned to measure blood pulsing in portions of theprinceps pollicis artery, which is the principal artery of the thumb andstems from the radial artery. Measuring blood flowing in this arteryenhances the accuracy of a PTT-based measurements of blood pressure, asis described in more detail in VITAL SIGN MONITOR FOR MEASURING BLOODPRESSURE USING OPTICAL, ELECTRICAL, AND PRESSURE WAVEFORMS (U.S. Ser.No. 12/138,194; filed Jun. 12, 2008 and published as 20090018453A1), andBODY-WORN SYSTEM FOR MEASURING CONTINUOUS NON-INVASIVE BLOOD PRESSURE(cNIBP) (U.S. Ser. No. 12/650,354, filed Nov. 15, 2009 and published as20100168589A1), the contents of which have been previously incorporatedby reference.

Electronic circuitry providing the LEDs and photodetector may beprovided in the form of a small flexible circuit board 109 in the sensormodule, which may additionally include, for example, a trans-impedanceamplifier for amplifying photocurrent from the photodetector andconverting it into a corresponding voltage. The electronic circuitry canalso include a resistor that identifies specific wavelengths emitted bythe LEDs; these wavelengths, in turn, influence values of correlationcoefficients that relate RoR to SpO2, as described below. In otherembodiments, the flexible circuit board 109 can be replaced with aseries of insulated wires that connect the LEDs, photodetector,trans-impedance amplifier, and other components to form a circuit.

A flexible housing 110 covers and protects the flexible circuit board109. The flexible housing 110 may preferably be formed from any waterand alcohol-resistant material, with materials having a durometer (A) inthe range of 25 to 60 (e.g., HTV, RTV, or LSR elastomers, etc.)providing an optimum combination of comfort and sufficient resistance tocompression to allow the probe to apply pressure to the underlyingarteries. A particularly preferred material has a durometer (A) of about40, such as Nusil MED-4044 silicone elastomer. For purposes ofmanufacturing, the circuit board may be inserted into the housingthrough a slit-like opening 115 at the top, which may then be sealed,for example with a liquid silicone elastomeric material. A connectioncable 116 extends through an opening 117 at the bottom of the housing.

The housing features rectangular openings 111 and 112 through which theLEDs emit, and the photodetector receives, radiation. In certainembodiments a filter may be provided over the photodetector in order toboth seal the aperture and filter undesired ambient light frequencies.On the emission side, the opening 11 may be sealed using a siliconematerial which is sufficiently transparent at the emission wavelengths.Forming the housing in a white color allows the housing material toreflect more of the scattered radiation from the sources to thedetector, thereby improving sensor efficiency. The size of openings 111and 112 also affect the efficiency of the sensor, with the optimal sizebeing windows that are somewhat larger than the active component areas.Preferred window sizes are from about 3 to 5 mm, and may be of any shapeincluding square, rectangular, or circular.

The housing also features two cut-out portions 113, 114, or ‘livinghinges’, that make it easily bendable and able to accommodate to theshape of the cradle module. These living hinges serve to subdivide thecradle into three substantially planar sensor module surfaces. A firstplanar surface comprises the radiation sources; a middle planar surfaceprovides a flat base surface to engage the thumb tissue of the subject;and a third planar surface comprises the photodetector. As shown in FIG.4, when mated to the cradle, the housing substantially conforms to theshape of the cradle, while providing three relatively planar surfaceswhich contact the tissue. These surfaces provide for a more uniformpressure application to the underlying arteries, thereby helping toincrease signal amplitudes and maintain a consistent light path. Thisalso serves to improve sensor efficiency.

To ensure proper registration of the sensor module within the cradlemodule, the surfaces of each which come into contact when mated comprisecorresponding registration structures, e.g., a ridge structure on thesensor module which inserts into a corresponding dimple or opening 118,119 on the cradle module. The registration structures may take the formof a protruding ridge disposed on a surface of the sensor module housingwhich opposes the openings 111, 112. When the sensor module is properlypositioned, the ridges insert into the openings 118, 119 in the cradlemodule to provide positive registration of the two modules. The skilledartisan will understand that other arrangements of registrationstructures may be probided. For example, the protruding ridge may bedisposed on a surface of the cradle module, and a corresponding dimplemay be disposed on the sensor module. The distal portions of the sensormodule may also insert into registration slots 121, 122 in the cradlemodule, thereby further ensuring proper positioning of the sensor modulewithin this component.

The probe is held in place around the base of the digit with areclosable retainer, such as a flexible nylon strap 120. The strap canbe attached to the cradle in a number of methods known in the art, suchas threading through two openings located the cradle's distal ends; orby bonding of the cradle to the strap via an adhesive, heat-staking orultra-sonic welding as depicted in FIG. 4. A portion of the strapfeatures a patch of Velcro (containing, e.g., ‘hooks’) that adheres to amated patch (containing, e.g., ‘loops’) on the strap's main portion; thepatches reversibly adhere to each other when the probe is placed on thepatient's digit, and easily detach so that it can be removed. The strapallows the probe to be securely fastened, which in turn minimizes motionrelative to the measurement site.

The flexible cable 116 serves to connect the electronics of the oximeterprobe to an external processor for processing signals necessary todetermine SpO2 and/or blood pressure measurements. The cable can carryI/O signals to the probe electronics that drive the LEDs according to atiming diagram, and analog signals measured by the photodetector to anamplifying/filtering circuit. The cable preferably comprises a distalconnector 123 that plugs into an external processing module to operablyconnect the probe to an external processor. There, the analog signalsare amplified, filtered, digitized, and processed to measure SpO2 and/orblood pressure (e.g. cNIBP) values, as described in detail below.

A detailed description of a preferred embodiment for the circuitry to beincorporated into the oximeter probe, and interconnection of theoximeter probe to a wrist-worn external processing module may be foundin International Patent Application No. PCT/US2010/039000 entitledBODY-WORN PULSE OXIMETER, which is hereby incorporated by reference inits entirety.

FIG. 5 depicts in schematic form one example of the sensor module beinginserted into the cradle module prior to use. For insertion, the sensormodule is flexed at living hinges so that it may approximate thesemicircular shape of the cradle module. As noted above with regard toFIG. 2, the sensor module contains the electronic components of thephysiological probe, and is configured for multiple uses. In contrast,as depicted in FIG. 1, the cradle lacks electronic components and isintended for disposable use as it contains parts which may be moredifficult to properly clean. Because of the rigid structure andconstrained hinge of the cradle module, insertion of the sensor moduleinto the cradle module serves to properly position the LEDs relative tothe photodetector for an accurate pulse oximetry measurement.

During a pulse oximetry measurement the LEDs intermittently emit beamsof radiation at 660 nm and 905 nm at roughly 500 Hz according. Onceemitted, the beams pass into the underlying tissue and rapidly divergeto scatter off tissue such as skin, bone, and capillaries near thetissue's outer surface. In the case of the thumb, a portion of theprinceps pollicis artery is also sampled by the radiation before itilluminates the photodetector. Both the capillaries and the arteriescarry blood that pulsates with each heartbeat and absorbs radiationemitted by the LEDs. This results in separate time-dependent opticalwaveforms (i.e. RED/IR(PPG), generated by the 660 and 905 nm radiation.Both waveforms feature AC signals corresponding to the time-dependentpulsating blood, and DC signals corresponding to time-independentscattering off the skin, bone, and non-pulsating components of thecapillaries and artery. Prior to any filtering the AC componenttypically represents about 0.5-1% of the total signal.

Collectively processing both the AC and DC signals of the RED/IR(PPG)waveforms yields a SpO2 value. The external processor can calculatethese components using a number of signal-processing methodologies thatare additionally important for determining PTT-based cNIBP. Ultimatelythe AC and DC components yield a RoR which then relates to a SpO2 usinga series of empirically determined coefficients. In one embodiment, forexample, the RoR is determined by first measuring RED/IR(PPG) waveforms,and then passing them through a low-pass filter characterized by a 20 Hzcutoff. The averaged baseline components of each waveform are sampledand stored in memory, and represent RED/IR(DC). Both waveforms are thenadditionally filtered with high-pass filter having a 0.1 Hz cutofffrequency, which is typically implemented with a finite impulse responsefunction, and finally amplified with a variable gain amplifier. Thesesteps can be done with either analog electronic or digital softwarefilters. Once determined, the AC and DC signals are processed to yield aRoR value, described in equation (3), which relates to SpO2:

$\begin{matrix}{{RoR} = \frac{{{RED}({AC})}/{{RED}({DC})}}{{{IR}({AC})}/{{IR}({DC})}}} & (3)\end{matrix}$

FIG. 6 shows an empirical relationship between RoR and SpO2 formeasurements made at the base of the thumb with the oximeter probe,along with similar relationships for measurements made at the tip of theindex finger with an off-the-shelf oximeter probe (small dashes), andthe theoretical curve for measurements made from the tip of the indexfinger (large dashes). Curves corresponding to measurements made fromthe index finger and thumb are determined empirically from a group ofpatients measured under similar conditions. As is clear from the figure,the relationships between RoR and SpO2 are similar, but slightly offsetdue to differences in the measurement site. Without being bound to anytheory, these differences may be due to the relatively low density ofcapillary beds near the base of the thumb as compared to those in thetip of the index finger. The relationship for all curves in FIG. 6 isnon-linear, particularly for SpO2 values ranging from about 70-100%.Values below 70% can be accounted for with a different non-linear model,such as one based on a second-order polynomial. Coefficients a, b, and cfor this model are determined by fitting the empirical data to acorresponding mathematical function like the second-order polynomialshown in equation (4) below:SpO2=(a+b*RoR+c*RoR²)×100  (4)

Optimized values for a, b, and c coefficients corresponding tomeasurements made at the base of the thumb are shown in Table 1, below:

TABLE 1 coefficients for equation 4 relating RoR to SpO2 formeasurements made at the base of the thumb Parameter Value A 107.3 B−3.0 C −20.0

The exact values of parameters shown in Table 1 will depend of thespecific wavelengths of the LEDs used in the pulse oximeter probe. Thisis because the SpO2 measurement is fundamentally determined by therelative optical absorption of Hb and HbO2 in the red and infraredspectral regions, and absorption, in turn, will depend on the wavelengthemitted by the LEDs. The absorption spectra of Hb and HbO2 arerelatively flat in the infrared spectral region, but strongly diverge inthe red spectral region. The coefficients shown in Table 1 are thusrelatively sensitive to the exact wavelength of the red LED. For thisreason, a series of empirical studies need to be performed using pulseoximeter probes featuring LEDs of varying wavelengths surrounding thered emission wavelength (e.g. 600-610 nm) prior to manufacturing. Such astudy is typically classified as a ‘breathe down’ study because itinvolves lowering the SpO2 values of a series of patients (typicallyabout 10-15) under medical supervision. SpO2 is typically lowered bydecreasing the amount of oxygen each patient inhales through aspecialized ventilator mask; this is often done in a room with a reducedtemperature. Blood from the patients is extracted from an arterial lineand analyzed with a blood gas analyzer to determine its oxygen content.Simultaneously, a pulse oximeter probe with known LED wavelengths isattached to each patient (in this case at the base of the thumb) andused to measure the RoR described in equation (3). SpO2 values for thisexperiment, as measured with the blood gas analyzer, typically rangefrom 70-100%. Simultaneous studies are typically done using pulseoximeter probes having LEDs with different red emission spectra. Uponcompletion of the studies, the wavelength-dependent values of RoR arerelated to SpO2, as determined by the blood gas analyzer, to calculatecoefficients a, b, c as described in Table 1. In general, a differentset of coefficients will result for the different LED wavelengths. Thesecoefficients and the optical wavelengths they correspond to, along witha resistor value described below, are stored in a database in memory onthe wrist-worn transceiver.

Measurements made at the base of the thumb provide improvements to themeasurement of SpO2 values because the circulatory anatomy at thislocation yields a more predictable arterial signal; this location alsoincreases patient comfort by freeing up the tips of the fingers for use.The IR(PPG) measured from this site, when processed in combination tothe ECG waveform, yields a PTT value that can be processed with theComposite Technique to yield an accurate cNIBP measurement. As describedabove, an IR(PPG) waveform measured from primarily from the princepspollicis artery increases the accuracy of the cNIBP measurement. With aninitial pressure-based calibration systolic (SYS) and diastolic (DIA)blood pressure can be explicitly determined for each heartbeat using analgorithm described in the above-mentioned patent applications, thecontents of which have been previously incorporated herein by reference.Typically, PTT values are processed over a 20-40 second time period(often implemented as a ‘rolling average’) using statistical filteringto improve accuracy. To better define the onset of the PPG waveform, andthus improve the accuracy to which SYS and DIA are determined, timescorresponding to RED(foot) and IR(foot) are typically averaged together.When compared to SYS and DIA values measured under clinical conditionswith a femoral arterial line, cNIBP measurements made from thisparticular location were well within the FDA's standards for accuracy(+/−5 mmHg) and standard deviation (8 mmHg). For this and other reasonsthe base of the thumb appears to be a uniquely good location formeasuring both SpO2 and cNIBP.

One skilled in the art readily appreciates that the present invention iswell adapted to carry out the objects and obtain the ends and advantagesmentioned, as well as those inherent therein. The examples providedherein are representative of preferred embodiments, are exemplary, andare not intended as limitations on the scope of the invention.

It will be readily apparent to a person skilled in the art that varyingsubstitutions and modifications may be made to the invention disclosedherein without departing from the scope and spirit of the invention.

All patents and publications mentioned in the specification areindicative of the levels of those of ordinary skill in the art to whichthe invention pertains. All patents and publications are hereinincorporated by reference to the same extent as if each individualpublication was specifically and individually indicated to beincorporated by reference.

The invention illustratively described herein suitably may be practicedin the absence of any element or elements, limitation or limitationswhich is not specifically disclosed herein. Thus, for example, in eachinstance herein any of the terms “comprising”, “consisting essentiallyof” and “consisting of” may be replaced with either of the other twoterms. The terms and expressions which have been employed are used asterms of description and not of limitation, and there is no intentionthat in the use of such terms and expressions of excluding anyequivalents of the features shown and described or portions thereof, butit is recognized that various modifications are possible within thescope of the invention claimed. Thus, it should be understood thatalthough the present invention has been specifically disclosed bypreferred embodiments and optional features, modification and variationof the concepts herein disclosed may be resorted to by those skilled inthe art, and that such modifications and variations are considered to bewithin the scope of this invention as defined by the appended claims.

Other embodiments are set forth within the following claims.

What is claimed is:
 1. A physiological probe configured for securing toa subject's digit, comprising: a sensor module comprising (a) electroniccircuitry which comprises (i) at least one source of electromagneticradiation, and (ii) a photodetector configured to detect radiation fromthe at least one source of electromagnetic radiation after it hasinteracted with the subject's tissue, thereby acquiring data relating toat least one physiologic property of the subject, wherein thephotodetector is operably connected to a connection cable for deliveryof photodetector signals to an external processing unit, and (b) aflexible enclosure providing a cover for the electronic circuitry,wherein the flexible enclosure comprises (i) at least one aperturethrough which the at least one source of electromagnetic radiation emitsradiation for irradiating the subject's tissue, (ii) at least oneaperture through which the photodetector receives radiation after it hasinteracted with the subject's tissue, and (iii) an aperture throughwhich the connection cable passes; and a cradle module comprising (a) atleast first and second rigid housing members mated to one another via ahinge region to form an approximately semicircular ring which isconfigured to releasably receive the sensor module, wherein the hingeregion is configured to provide, when secured to the subject's digit, arange of adjustment in a diameter of the ring while physicallyconstraining an included angle measured between the at least one sourceof electromagnetic radiation and the photodetector within apredetermined range, wherein the predetermined range of the includedangle measured between the source(s) of electromagnetic radiation andthe photodetector is between about 60° and about 30°.
 2. A physiologicalprobe according to claim 1, wherein the hinge is configured toaccommodate a subject's thumb at the level of the proximal phalanx forpurposes of acquiring the data relating to at least one physiologicproperty of the subject.
 3. A physiological probe according to claim 1,wherein the hinge is configured to accommodate a girth of between about6.2 cm and about 7.4 cm.
 4. A physiological probe according to claim 1,wherein the hinge region is a pivoting hinge formed by mating the firstand second rigid housing members, and wherein the structure of the firstand second rigid housing members constrains opening or closing of thehinge beyond the predetermined range when mated to one another via thepivoting hinge.
 5. A physiological probe according to claim 4, wherein,the hinge is configured such that a force applied to move the hingebeyond the predetermined range separates the first and second rigidhousing members.
 6. A physiological probe according to claim 1, whereinthe predetermined range of the included angle measured between thesource(s) of electromagnetic radiation and the photodetector is betweenabout 55° and about 35°.
 7. A physiological probe according to claim 1,wherein the sensor module comprises one or more hinge points in theflexible enclosure.
 8. A physiological probe according to claim 7,wherein the sensor module comprises at least two hinge points in theflexible enclosure configured to provide three substantially planarsensor module surfaces when the sensor module is mated to the cradlemodule.
 9. A physiological probe according to claim 1, wherein thesurface of the sensor module and the surface of the cradle module whichcome into contact when the two modules are mated comprise correspondingregistration structures configured to register the position of thesensor module in the cradle module.
 10. A physiological probe accordingto claim 1, wherein the length of the cradle module is between about 1cm and about 1.5 cm.
 11. A physiological probe according to claim 1,wherein the cradle module is disposable.
 12. A method of obtaining pulseoximetry signals from a subject, comprising: affixing a physiologicalprobe according to claim 1 to the palmar aspect of a digit of thesubject at the level of the proximal phalanx, wherein the probe ispositioned on the subject's digit such that the source(s) ofelectromagnetic radiation, when energized, provide irradiation oftissues of the subject through the capillary, arterial, and soft tissueof the digit, and subsequently to the photodetector; energizing thesources to irradiate the tissues of the subject; and detectingphotodetector signals at an external processing unit operably connectedvia the connection cable.
 13. A method according to claim 12, furthercomprising calculating one or more of a blood pressure value, an SpO2value, a pulse rate, a respiration cycle, and a photoplethysmographwaveform for the subject using the photodetector signals.
 14. A methodaccording to claim 12, comprising determining a blood pressure value forthe subject using the photodetector signals.